System for detecting zero-field resonance

ABSTRACT

A zero-field paramagnetic resonance magnetometer (ZF-PRM) system and method for quickly and efficiently finding and optimizing the zero-field (ZF) resonance is described. In this system and method a magnetic coil is used to apply a magnetic bias field in the direction of the pump beam to artificially broaden the width and maximize the strength of the ZF resonance. By making the ZF resonance easy to detect, the ZF resonance may be found quickly found without the use of additional components and complex algorithms. Once the ZF resonance is found, a compensating magnetic field can be applied to null the magnetic field in the vicinity of the vapor cell in the ZF-PRM, thereby initializing it for operation.

REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/965,455 filed Jan. 30, 2014.

FIELD OF INVENTION

This disclosure relates to the field of paramagnetic resonancemagnetometers and in particular a magnetometer and an accompanyingmethod for detecting background magnetic fields. Further, the disclosurerelates to a method for compensating for the background magnetic fieldand measuring a target magnetic field.

BACKGROUND

High sensitivity magnetometers, including paramagnetic resonancemagnetometers (PRM) (Slocum & Reilly, 1963), have a wide range ofapplications including, but not limited to the following: fundamentalresearch, detecting biomagnetic signals (such as those emanating frombiological organisms, including the human body), geophysical explorationand prospecting, navigation and space applications, and military uses(such as ordinance and underground-underwater structure detection).Until recently, the most sensitive and commercially availablemagnetometers for such applications were based on superconductingquantum interference devices (SQUID) (Weinstock, 1996). However,zero-field paramagnetic resonance magnetometers (ZF-PRM); (Dupont-Roc,Haroche, & Cohen-Tannoudji, 1969; Marie-Anne et al., 1971; Shah, Knappe,Schwindt, & Kitching, 2007; W & E, 1974), which have advanced tocomparable sensitivity as SQUID systems, have recently gained popularityas a lower-cost, more robust alternative to SQUID magnetometers for manyapplications. The current rapid development and commercialization ofsuch atomic based magnetic sensors may lead to replacement of SQUIDbased sensors for many existing applications primarily because ZF-PRMsdo not require cryogenic cooling.

Significant developments in alkali atomic magnetometery (Budker &Romalis, 2007) over the past decade have led to a variety of techniquesand methods for sensing magnetic fields. In general, the differentmethods are based on the same fundamental physical sensing mechanismthat exploits the energy structure of atoms and the perturbations thatresult in their energy levels (or spin state) from exposure to externalmagnetic fields. In essence, atomic based magnetic sensors measure thedirection and magnitude of an external magnetic field through theinduced changes in the atomic spin polarization of an ensemble of atoms.

A ZF-PRM relies on detecting changes in optical transmission propertiesof atomic vapor around a narrow ZF atomic resonance to measure themagnitude and direction of the background magnetic field. Generally a ZFresonance can only be observed when the atomic vapor (the ensemble ofatoms) in the magnetometer is subjected to very small magnetic fields,generally less than 100 nanotesla (nT). Typically, the full width of theZF resonance is less than 30 nT. It is therefore necessary for theambient magnetic field at the location of ZF-PRM to be less than thedetection range of the magnetometer. The detection range of a ZF-PRM istypically a fraction (less than about 1, for example, one half) of thewidth of the ZF resonance. The magnetometer becomes less sensitive oreven insensitive when the total background magnetic field is greaterthan the detection range. For this reason, the ZF-PRMs are frequentlyused inside magnetically shielded environments in which the ambientmagnetic field is very small, typically few tens of nT or less. WhenZF-PRM is used in an unshielded or poorly shielded environment where alarge background magnetic field is present, external biasing coils areused to null the magnetic field in vicinity of the ZF-PRM, keeping themagnetometer within its detection range. This operational requirement ofnear ZF condition limits the utility of ZF-PRMs for many applications;this includes biomedical applications, such as Magnetoencephalography(MEG) and Magnetocardiography (MCG) where large and expensive shieldedvolumes are required, as well as any outdoor applications where magneticshielding is impractical.

SUMMARY OF THE INVENTION

This invention relates to the use of a ZF-PRM in any magnetic fieldenvironment, including in unshielded magnetic field environments, usingan apparatus and accompanying methods provided herein to actively nullthe background or ambient magnetic fields. The act of nulling a field,wherein the field is effectively equal to zero, can occur when the widthof the measured resonance is minimized and height is maximized, and istherefore said to be optimized. This nulling may be generated using aset of external coils (to compensate in all three axes) that produce thedesired opposing magnetic fields. Precisely zeroing the magnetic fieldis a complex and cumbersome task and often requires precise a prioriknowledge of the ambient field. The apparatus and method presented hereoffers a solution to zeroing the field without a priori knowledge of theambient field with the additional benefit of lower cost and timeexpenditures compared to other methods.

We note that the present apparatus and method solves a long-standingcomplexity which has been recognized by those skilled in the art ofbuilding and operating ZF-PRMs. To highlight the importance of ourmethod, we include here text from recent prior art by leaders in fieldof ZF-PRM who are skilled in the art:

“We find that the easiest method for zeroing the magnetic field is touse a different sensor, such as a fluxgate, and then turn on feedbackfrom the magnetometer once the field along all three directions issufficiently small. Zeroing the field using only the magnetometer signalis extremely difficult and inefficient, especially when the fieldamplitude is much larger than the magnetic linewidth. Once feedback isactive, the magnetometer is able to track changes in the localenvironment, such as motion of distant magnetic objects.” (S. Seltzer,2008, “Developments in Alkali-Metal Atomic Magnetometry”, PhDDissertation, Ch. 5, Pg. 156)

“Because the earth field amplitude is much larger than the dynamic rangeof atomic vector magnetometer, zeroing the field using only atomicsignal is extremely difficult and inefficient.” (Haifeng Dong, Lin, &Tang, 2013, “Atomic-Signal-Based Zero-Field Finding Technique forUnshielded Atomic Vector Magnetometer”, IEEE Sensors Journal, 13(1),186-189)

“With the magnetic field unknown, it is difficult to tell whether thecondition has been satisfied or not, so it is not convenient to use thismethod for triaxial magnetic field compensation. This is why fluxgatemagnetometers are usually used to measure the triaxial magnetic fieldsto guarantee operation of the SERF atomic magnetometer under magneticshield room conditions.” (Fang & Qin, 2012, “In situ triaxial magneticfield compensation for the spin-exchange-relaxation-free atomicmagnetometer”, The Review of Scientific Instruments, 83(10), 103104)

In general, the natural ZF resonance is not only very narrow, but occurswhen the magnitude of the magnetic field is very close to zero. Becausethe earth's magnetic field [˜50 microtesla (μT)] is many orders ofmagnitude larger than the width of the natural ZF resonance (˜30 nT orless), locating the natural ZF resonance by simply scanning the nullingfield in all three directions without prior knowledge of the magnitudeand direction of the background field is akin to finding a needle in ahaystack.

To scan through all possible field values in steps of 15 nT increments,i.e. to insure the detection of the resonance signal, in each of thethree axial directions would require (50 μT/15 nT)^3 or 37,037,037,037,roughly 37 trillion steps. It is thus a very time intensive task to findthe ZF resonance condition in an unknown magnetic field environmentwithout the use of external aids to provide some information about themagnitude and direction of the ambient magnetic field.

Algorithms and techniques have been developed to scan for the ZFresonance more effectively. Such methods are considerably complex, timeconsuming, and impractical for many applications.

As eluded to above, a prior art method uses a separate high-performancetriaxial fluxgate magnetometer to accurately measure the magnitude anddirection of the background magnetic field and then apply compensatingfields to null the total ambient magnetic field at the location of thefluxgate (S. Seltzer, 2008; S. J. Seltzer & Romalis, 2004). Afterapproximately zeroing the field, the fluxgate magnetometer is removedand replaced by ZF-PRM. This method significantly reduces the scan rangeand time necessary to find the ZF resonance. However, this method alsoadds additional equipment and complexity, and therefor expense, to thesystem. The ZF-PRM as well as a fluxgate magnetometer may cost at leastseveral thousand dollars each, such that addition of the fluxgatemagnetometer may more than double the cost of the system. In addition,the fluxgate magnetometers often have intrinsic magnetism, which is whythey have to be physically removed and placed away from ZF-PRM when theZF-PRM is in operation.

Another technique for nulling the background field using ZF-PRM alonewas developed by (Haifeng Dong et al., 2013), who used an iterativeconvergence algorithm based on an approximate prior knowledge of themagnitude and direction of earth's magnetic field. Fang & Qin, 2012,have used another iterative convergence algorithm that works underfavorable initial conditions such that the transverse field components(Bx and By) are not much greater than the longitudinal field component(Bz). As is illustrated in FIG. 1, Bx and By is the magnitude of themagnetic field components in a direction perpendicular to a pump beam20, as is described in more detail in the discussion of FIG. 1,following, and Bz is the magnetic field parallel to the pump beam 20.

What is needed in the art is a simple, quick, and inexpensive method offinding and nulling the ambient magnetic field such that small fieldchanges may be measured in any environment. The present invention solvesthis problem, being a system and method that provides such a simple,inexpensive, and fast method for zeroing the magnetic field without apriori knowledge of the background field.

This example system and method is a novel system and method for findingparamagnetic resonance in an unknown magnetic field. Summarizing theoperation of a ZF-PRM 101, illustrated by FIG. 1, the behavior of thespin polarization vector ({right arrow over (P)}) of alkali atoms 60 ina sensing cell 70 can be understood based on the Bloch equation:

$\begin{matrix}{{\frac{d\overset{\rightarrow}{P}}{d\; t} = {{\gamma\left( {\overset{\rightarrow}{B} \times \overset{\rightarrow}{P}} \right)} - {R\left( {\overset{\rightarrow}{P} - {\overset{\rightarrow}{P}}_{0}} \right)}}},} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$where γ is the gyromagnetic ratio of the alkali atoms 60, R is thecombined optical pumping and relaxation rate, {right arrow over (P)}₀ isthe equilibrium spin polarization of alkali atoms 60, and {right arrowover (B)} is the magnetic field to which alkali atoms are exposed. Arepresentation of a magnetic field may be detected as a change intransmission of the light or pump light beam through the alkali atoms bya photodetector. The amount of light beam 20 transmitted is proportionalto the steady state solution of the said equations along thez-direction, i.e.:

$\begin{matrix}{{P_{z} = {P_{0}\frac{B_{z}^{2} + \left( \frac{R}{\gamma} \right)^{2}}{B_{x}^{2} + B_{y}^{2} + B_{z}^{2} + \left( \frac{R}{\gamma} \right)^{2}}}},} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$where P_(z) is the component of the spin polarization vector, {rightarrow over (P)}, in the z-direction.

When By (or Bx) and Bz is near zero, a change in the amount of lighttransmitted through the vapor cell can be observed when the Bx (or By)field is scanned about the ZF value. This change in transmissionproperties of the vapor cell is referred to as ZF resonance and itswidth is equal to

$\left( \frac{R}{\gamma} \right),$which is on the order or about 30 nT or less. As illustrated in FIG. 2,ZF resonance R is expressed as a signal having height, H, and width, W.The derivative of the resonance produces a dispersion curve which werefer to herein as the error signal E. The derivative peak E is used tolock the field values as will be described later. ZF-PRM with differentconfigurations, such as those illustrated in FIG. 3 and FIG. 4, may havedifferent shaped resonance curves, which may look for example likesignal E, or another shape, which will still have a height and widthvalue, the height and width being measured from peak to peak.

When By and Bz fields are non-zero, the width of the resonance, W, inthe x-direction is given by:

$\begin{matrix}{W = \sqrt{B_{y}^{2} + B_{z}^{2} + \left( \frac{R}{\gamma} \right)^{2}}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$and the height, H, of the ZF resonance is proportional to, P_(z), and isgiven by:

$\begin{matrix}{{H \propto P_{z}} = {P_{0}\frac{B_{z}^{2} + \left( \frac{R}{\gamma} \right)^{2}}{B_{y}^{2} + B_{z}^{2} + \left( \frac{R}{\gamma} \right)^{2}}}} & \left( {{Eq}.\mspace{14mu} 4} \right)\end{matrix}$

From Eq. 4, we see that when By is significantly greater than Bz, theamplitude of the resonance is significantly diminished and may not beobservable by simply scanning the magnetic field in x-direction alone.Consequently, attempting to find the ZF resonance without priorknowledge of the magnitude and direction of the background field can bea cumbersome task.

In the example system and method of the present invention, a strongmagnetic field, or bias field, Bz^(†), is applied to the vapor cellalong the direction of the light beam used to pump atoms in the vaporcell. As an example, the value of the bias field, may be less than thevalue of the background magnetic field in which the ZF-PRM is designedto operate, or preferably at least as great as the value of thebackground field in which the ZF-PRM is designed to operate, or at leastgreater than the value of the background field in which the ZF-PRM isdesigned to operate, or even as much as at least twice the maximum valueof the background field in which the ZF-PRM is designed to operate, ormore. Applying a strong bias field Bz^(†) ensures that the conditionBy >>Bz or Bx>>Bz is never met regardless of the orientation of themagnetometer with respect to the ambient field, and therefore, theamplitude of a resonance peak will not be diminished by the presence ofany transverse field. Additionally, the width of the resonance peakgreatly increases from applying a bias field, Bz^(\), as seen from Eq.2. To summarize, the method of applying a bias field, Bz^(\), presentedherein, ensures that the ZF resonance is both wide and strong inamplitude, or height, such that locating the ZF resonance isstraightforward, fast, and requires no additional equipment, or complexalgorithms.

Once the ZF resonance is detected, compensating fields can be applied ina way that results in a ZF resonance with minimum width and maximumamplitude, i.e. height. For ZF resonance in x-direction, the value ofthe compensating magnetic field that produces minimum width and maximumamplitude is the point at which the magnetic field is closest to zero atthe location of the vapor cell in y-direction and z-direction. Thecompensating magnetic field value corresponding to the peak of theresonance gives magnetic field closest to zero at the location of thevapor cell in the x-direction. The ZF-PRM is maximally sensitive whenmagnetic field is nearly zero in all three directions at the location ofthe vapor cell.

As such an example method for identifying and optimizing a ZF resonanceusing a paramagnetic resonance magnetometer, is described and claimedherein. The example method comprises the steps of: directing at leastone pump light beam through a vapor cell containing gaseous atoms toincrease the magnetic polarization of the gaseous atoms; measuring lighttransmitted through the vapor cell; in a non-iterative action, applyinga strong magnetic field having a direction along the pump light beam tosimultaneously increase the height and width of the ZF resonance, andsubsequently detecting the ZF resonance; scanning a magnetic field in adirection differing from that of the pump light beam; and adjustingmagnetic field components generated by one or more coils to minimize thewidth and maximize the height of the zero-field resonance.

The strong magnetic field applied along the direction of the pump lightbeam may be at least about as strong as a background magnetic field, oreven stronger than the background magnetic field. Alternately, thestrong magnetic field applied along the direction of the pump light beammay be weaker than the background magnetic field.

The gaseous atoms in the vapor cell may be selected from a groupconsisting of rubidium, cesium, potassium, sodium, and helium. Further,the magnetometer may be oriented along any arbitrary direction withrespect to a background magnetic field. Additionally, the magnetometerneed not be placed within a magnetic shield.

The scanning of the magnetic field in a direction differing from that ofthe pump light beam, occurs within a range, the scan range may be atleast about 0.1 times as strong, or at least about 0.5 times as strong,or at least about as strong as the magnetic field in the direction ofthe pump light beam. Further, the scan range may be as much as 2 times,or even as much as 5 times, or even as much as 10 times as strong as themagnetic field in the direction of the pump light beam.

The example method, further enhanced for precision offset fieldgeneration, may further comprise the step of adding a calibrated field.The calibrated field may be generated by one or more coils that may beinternal or external to the magnetometer. Further, the one or more coilsmay be an additional single coil, or an additional two coils, or set ofcoils, or the sole coil, or coils, or set of coils of the magnetometer.

A second example for locking a zero-field paramagnetic resonancemagnetometer (ZF-PRM) to a ZF resonance is presented and claimed herein.This second example method comprises steps of directing at least onepump light beam through a vapor cell containing gaseous atoms toincrease the magnetic polarization of the gaseous atoms; measuring lighttransmitted through the vapor cell; in a non-iterative action, applyinga strong magnetic field having a direction along the pump light beam tosimultaneously increase the height and width of the ZF resonance;applying a modulation current having an amplitude to at least one coiltransverse to the pump light beam; generating at least one error signal;and subsequent to applying the strong magnetic field having a directionalong the pump light beam, engaging at least one control loop tominimize at least one field component that is transverse to the pumplight beam.

The strong magnetic field applied along the direction of the pump lightbeam is at least about as strong as a background magnetic field, or evenstronger than the background magnetic field. Alternately, the strongmagnetic field applied along the direction of the pump light beam may beweaker than the background magnetic field.

The gaseous atoms in the vapor cell may be selected from a groupconsisting of rubidium, cesium, potassium, sodium, and helium. Further,the magnetometer may be oriented along any arbitrary direction withrespect to a background magnetic field. Additionally, the magnetometerneed not placed within a magnetic shield.

Further, the modulation current may be applied to at least two coilstransverse to the pump light beam, such that at least two error signalsare generated. The modulation current may be applied to at least threecoils transverse to the pump light beam, such that at least three errorsignals are generated.

This example method for locking a ZF-PRM to a ZF resonance may furthercomprise the steps of changing the amplitude of at least one modulationcurrent and optimizing at least one error signal.

Inasmuch, this example method for locking a ZF-PRM to a ZF resonance,may yet further comprise the step of engaging at least one control loopthat minimizes the field component in the direction along the pump beam,this step being subsequent to the step of engaging at least one controlloop to minimize at least one field component that is transverse to thepump light beam.

The method may be further enhanced for precision offset field generationand further comprise deactivating one or more control loops, andsubsequently adding a calibrated field. The calibrated field may begenerated by one or more coils that are either internal or external tothe magnetometer. Further, the one or more coils may be an additionalsingle coil, or an additional two coils, or set of coils, or the solecoil, or coils, or set of coils of the magnetometer. Even so, the methodmay further comprise adding one or more calibrated offsets to one ormore control loops.

For the purposes of the present application the term “field” when notaccompanied by a qualifier is defined to mean magnetic field. The terms“large” or “strong” when referring to a field are defined as a fieldwith a magnitude having a range at least about 0.1 times the backgroundfield to as much as about 10 times the background field. The terms“background” and “ambient” when qualifying a field are interchangeable.The term “background magnetic field”, or equivalently “background field”is defined to mean a field to which the magnetometer is subjected, butwhich was not applied by the user or the magnetometer operation. As oneexample, Earth's magnetic field would be considered part of thebackground field for a magnetometer placed outside. Other parts of thebackground field could include nearby magnetic objects like cars orbuildings. The term “bias field” is defined to be a magnetic field thatis applied by the user, typically with a coil, for the operation of themagnetometer. A field has both direction and magnitude and herein theterm “field component” refers to the field along a given direction.

Further, the term “coil” is known in the art and defined to be an objectthat can produce a tunable magnetic field. One example of a coil is awire that has been wound in a circular shape through which electricalcurrent is passed to produce a magnetic field. Other geometries of acoil could include, but are not limited to a Helmholtz coil pair, around solenoid, or a wire wrapped in rectangular-shaped windings. Hereinthe term “null” means to apply a field to cancel a component of thebackground field. As one example of the term “null”, an x-bias coil ispositioned with the axis of the coil along the x-direction. Continuingthe example, current is applied to the coil to generate a field, at adesired point, that is equal in magnitude and opposite in direction tothe background field at that point. Further continuing the example, thesum of the x-component of the background field with the x-component ofthe applied bias field is zero, thus the x component of the field hasbeen nulled.

Herein the term “modulation” is used to describe periodic variations ineither a field or an electrical current that drives a coil. Similarly,the term “modulate” is defined herein to be the act of apply suchmodulation. Specifically the terms “modulate” and “modulation” apply toperiodic variations that enable error signal generation, as one example,through the lock-in detection technique. The term lock-in detection isknown in the art. The term “scan” or “scanning” is used in conjunctionwith a parameter and is defined herein to mean the act of changing aparameter to make a manual (visual) or computer-aided observation. As anexample, one may scan the field to observe the shape of the zero-fieldresonance using an oscilloscope. The “scan” need not be periodic as isrequired with “modulation”. The term “optimizing a zero-field resonance”is used herein in to describe the minimizing the width and maximizingthe height of the ZF resonance. Further, the act of nulling a field,wherein in the field is effectively equal to zero, can occur when thewidth of the measured resonance is minimized and height is maximized,and is therefore said to be optimized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a prior art ZF-PRM system using asingle light beam.

FIG. 2 illustrates a resonance peak and a dispersion peak.

FIG. 3 schematically illustrates a prior art, parallel, dual light beamZF-PRM system.

FIG. 4 schematically illustrates a prior art, perpendicular, dual beamZF-PRM system.

FIG. 5 schematically illustrates an example manual system foridentifying and optimizing a ZF resonance.

FIG. 6 illustrates a ZF resonance peak and the subsequent broadening ofthe resonance peak due to increasingly amplifiying a bias field Bz^(\).

FIG. 7 illustrates a flow sheet for the method for optimizing the ZFresonance by manually adjusting magnetic field components.

FIG. 8 schematically illustrates an example electronic system forlocking a ZF resonance.

FIG. 9 illustrates a flow sheet for the method for electronicallyoptimizing the ZF resonance by applying a modulation current andengaging a control loop to optimize a field component.

DETAILED DESCRIPTION OF THE INVENTION

Paramagnetic resonance magnetometers as shown in FIG. 1, are known inthe art as described previously as in U.S. Pat. No. 3,629,697 toBouchiat et al., and U.S. Pat. No. 4,005,355 to Happer et al., bothincorporated in full herein by reference. More recently, U.S. Pat. No.8,212,556 to Schwindt et al., and U.S. Pat. No. 7,145,333 to Romalis etal. described a paramagnetic resonance magnetometer wherein two beamsare used, one pump beam and a second probing beam, being illustrated inFIGS. 3 and 4 respectively, and incorporated in full herein byreference.

The theory, construction, and operation of ZF-PRMs, whether employing asingle or multiple beams, is described in prior art (Marie-Anne et al.,1971; S. J. Seltzer & Romalis, 2004; W & E, 1974). FIG. 1 schematicallyillustrates a single beam ZF-PRM used to measure background magneticfield. Similar components are used whether a single or multiple beamsare employed, and regardless of the way the beams are configured,whether they are parallel as shown in FIG. 3, or perpendicular, as shownin FIG. 4. Additionally, the systems shown in FIGS. 1, 3, and 4 may bemodified by a person skilled in the art to create the example system andperform the example method as presented by the inventors herein.

As shown in FIGS. 1, 3, and 4, a ZF-PRM apparatus 101, comprises a cell70 containing an alkali metal vapor or helium 60. The cell 70, which canbe made of glass, or some other transparent material, can also include abuffer gas 80. The buffer gas may comprise a noble gas such as helium,argon, xenon or neon. Another gas such as nitrogen can also be used asbuffer gas 80. The buffer gas may comprise of a mixture of gases or asingle component gas. The cell 70 may be heated to an elevatedtemperature to provide a density of alkali metal atoms, which can rangefrom at least about 10⁷ cm⁻³ to at least about 10¹⁵ cm⁻³ or more. Theexact temperature to which the cell 70 is heated will depend, ingeneral, upon the atomic species (e.g. sodium, potassium, rubidium, orcesium) which is used in the apparatus. As an example, when the alkalimetal comprises rubidium-87, the cell 70 may be heated to about roomtemperature or up to about 200° C. The cell 70 may be heated by locatingthe cell within an oven (not shown) or by placing it within thermalproximity to a heating element.

In the ZF-PRM 101, a pump light beam 20, which can be generated by alaser 10, or a vapor lamp 10, is directed through a linear polarizer 40to linearly polarize the pump light beam 20. The linear polarizer 40 canbe omitted if the pump light beam 20 is already linearly polarized. Thepump light beam 20, which can have an optical power level of up to a fewmicroWatts (μW) or more depending upon the size and temperature of thecell 70, can be expanded and substantially collimated by one or morelenses 30. The pump light beam 20 can be expanded, for example, fromfraction of a millimeter (mm) to a size which fills a majority of theinternal volume of the cell 70, as shown in FIG. 1. The cell may havelateral dimensions of generally about one mm or larger.

After being expanded and substantially collimated by the lenses 30, thepump light beam 20 may be directed through an optical waveplate 50having a fast axis which is oriented at 45° with respect to a direction(e.g. vertical out of the plane of FIG. 1 or horizontal in the plane ofFIG. 1) of the linear polarization of the pump light beam 20. In thisway, the optical waveplate 50 converts the pump light beam 20, which wasinitially linearly polarized, into being circularly polarized. Thecircularly-polarized light in the beam 20 after passing through theoptical waveplate 50 can be either right-handed circularly-polarizedlight or left-handed circularly-polarized light. After being transmittedthrough the optical waveplate 50, the pump light beam 20 is directedthrough the cell 70 containing the alkali metal vapor 60. For example,if two light beams, a circularly polarized pump light beam 20 andlinearly polarized probe beam 21, are employed for instance in parallelfashion, as in FIG. 3, a beam splitter 150 may combine the pump lightbeam 20 and the probe beam 21 into a combined light beam 22 before theyare directed towards the cell 70. If two beams are directed at the cellin a perpendicular fashion, as illustrate in FIG. 4, a beam 20 may bere-directed as such by a mirror 120, and presented to the vapor cellalong with the probe beam 21.

The optical waveplate 50 functions as a quarter waveplate at thewavelength of the pump light beam 20, which is substantially equal tothe wavelength of a first or second D1 line atomic transition of thealkali metal vapor 60. D1 line is defined herein as a transition from an²S_(1/2) ground state to a m²P_(1/2) excited state of the alkali metalatoms in the vapor 60 where n and m are integers. The pump light beam 20need not be exactly on line center of the D1 transition, but can betuned off the line center and onto the wings of the D1 transition.

The buffer gas 80 (e.g. helium, neon or nitrogen) which is in the cell70 is useful to slow down the rate at which the atoms of the alkalimetal vapor 60 collide with the inner walls of the cell 70 which canagain randomize the spins of the alkali metal atoms. The buffer gas 80pressure in the cell 70 can be, for example, in a range between about 1torr and about 2000 torr. Special coatings on the inner walls of thecell 70, such as octadecyltrichlorosilane (OTS) or paraffin, may be usedin lieu of, or in addition to, buffer gases 80 to reduce spinrandomization from wall collisions.

The pump beam 20, after passing through the vapor cell 70, is collectedby a one or more photodetectors 90, which provide(s) a measure of theamount of light transmitted through the cell 70, or a measure of thepolarization state of the light transmitted through the cell 70. Varioustypes of photodetectors 90 may be used with detection capability in thewavelength range of the pump light beam 20. The output of thephotodetector may be subsequently amplified using suitable low noiseelectronic controllers 100 (not shown in FIGS. 3 and 4). The beam,having passed through the vapor cell, may be split, by a beam splitter150, and directed to multiple photodetectors 90, as shown in FIGS. 3 and4.

The ZF-PRM apparatus 101, the cell 70 alone, or a space or room wherethe ZF-PRM is housed, may be surrounded by one or more, or one or moresets of electrically activated magnetic coils 110, a set of coils beingthree coils, one generating fields in the x-plane, one generating fieldsin the y-plane, and one generating fields in the z-plane, that generatemagnetic fields opposing a background field, substantially cancellingout, zeroing, nulling, or optimizing the resonance of any magnetic fieldin the region around the cell 70. An electronics controller 100 (notshown in FIGS. 3 and 4) for the coils 110, or magnetic field componentmay be programmed or manually operated to modify the direction andmagnitude of the magnetic field produced by the coils in order to zeroor null the magnetic field therefor optimizing the ZF resonance at thelocation of the vapor cell 70.

When the alkali atoms 60 in the cell 70 are in a zero magnetic fieldenvironment, the circular polarization of the pump light beam 20produced by the optical waveplate 50 aligns the nuclear and electronspins of the individual alkali metal atoms in the alkali metal vapor 70from optical pumping process (Napper & Mathur, 1967). The opticalpumping process re-orients the spins of the individual alkali metalatoms so that they are in a magnetically-polarized state aligned alongthe direction of the pump light beam 20 (i.e. defined here in as thez-direction, as shown in FIG. 1).

The amount of pump light 20 transmitted by the vapor cell 70 andcollected by the photodetector 90 is proportional to the degree of spinpolarization of the alkali atoms 60 in the z-direction, Pz. The value ofPz is given by Eq. 2. When the cell 70 is in a ZF environment, i.e.Bx=By=Bz=0, scanning the magnetic field, Bx 140 for example, produces anarrow Lorentzian resonance, defined herein as the natural ZF resonance,which can be seen by monitoring the amplified output of thephotodetector 90 on an oscilloscope. A resonance R, as shown in FIG. 2,is referred to in the prior art as the ZF resonance. The resonance R, orany resonance, may be defined has having a height, H, and a width W. Itshould be understood that modification of the ZF-PRM to includeadditional probe light beams, for example illustrated in FIGS. 3 and 4,may change the shape of the resonance peak but these peaks may still bedefined by a height and a width.

The width W of the ZF resonance R in the x-direction is given by Eq. 3,and the height H or amplitude of the resonance is given by Eq. 4. Thederivative of the resonance R is a dispersion curve, referred to hereinas error signal E. It is well known in the art that an error signal Ecan be created by applying a modulation to a resonance R, for example bymodulating a magnetic field, and then using a lock-in detector fordemodulation. When By=Bz=0, the resonance has the smallest width (R/γ)and the largest amplitude P₀. Therefore the magnetometer is maximallysensitive when Bx=By=Bz=0.

As discussed above various algorithms and additional equipment areavailable for detecting the ZF resonance and directing the coils 110 toproduce an environment in which Bx=By=Bz=0 at the vapor cell 70.Alternately, the present example is a system and method for simply,inexpensively, and quickly detecting the ZF resonance and setting theelectrically activated magnetic field coils 110 in a way that Bx=By=Bz˜0in the neighborhood of the vapor cell 70, without the use of additionalequipment or complex and time consuming algorithms.

To describe the present example system and method, FIG. 5 schematicallyillustrates a manual system and method for identifying and optimizing aZF resonance. As such, a ZF-PRM 101, simply referred to as amagnetometer, may be placed in an environment with potentially non-zerobackground magnetic field. This ZF-PRM would necessarily encompasscomponents described above including one or more laser or lightsource(s) 10, that generates a beam 20, that may pass through any numbera beam splitters (not shown), polarizers 30, lenses 40, and/orwaveplates 50. The beam would pass through a cell 70 containing targetalkali atoms or helium 60, and optionally a buffer gas 80. One or more,or one or more sets of magnetic coils 540, 550 and 560, producingmagnetic field components in the x-, y- and z-direction, respectively,would be employed to null the ambient magnetic field in the regionaround the cell 70. This has the effect of minimizing the width, andmaximizing the height of the ZF resonance, and thereby optimizing the ZFresonance. The coils 540, 550 and 560, which may also be Hemholz coils,are electrically powered by a coil driver 570.

The magnetometer 101 may be placed in an arbitrary orientation withrespect to the background magnetic field BG 510. The system employs acoil driver 570 to produce a strong magnetic field Bz^(\) 530 along thedirection of the pump beam, for example using a Helmholtz coil pair 560.The value of the Bz^(\) 530, may preferably be at least equal to orgreater than the maximum field to which the ZF-PRM is expected to beexposed, although a smaller bias or longitudinal magnetic field may alsobe used.

From Eq. 3, the said bias field Bz^(\) 530 greatly increases the widthof the resonance, and also maximizes the amplitude or height of theresonance. As an example, in the limiting case in which Bz^(\) issignificantly greater than Bx, By, Bz, R/γ, the width of the ZFresonance becomes about equal to Bz^(\). In addition, the amplitude ofthe resonance, which is proportional to Pz, takes on its maximum valueequal to P₀ from Eq. 4.

FIGS. 6(a)-(d) illustrates this change in the width of the ZF resonanceas the bias field Bz^(\) 530 is applied and amplified. FIG. 6(a)illustrates the ZF resonance in a ZF environment, prior to applicationof the bias field 530. FIGS. 6(b)-6(d) shows plots of the observedmagnetic resonance at gradually increasing values of the bias fieldBz^(\) 530. As the bias field is amplified, the width of the ZFresonance increases, as shown in FIGS. 6(b), 6(c), and 6(d). Due to thisincrease in width, the ZF resonance signal may be found and identifiedvery easily by the non-iterative application of a strong bias fieldBz^(\) 530.

Initially if the magnetometer 101 was in a non-zero magnetic field, theZF resonance in plot FIG. 6(a) likely would not be observed, when thebias field Bz^(\) 530 is not present. The resonance would graduallyappear in subsequent plots FIGS. 6(b)-6(d), with the resonance becomingwider and stronger as the bias field Bz^(†) 530 is applied and increasedin value.

By scanning the magnetic field in a direction differing from that of thepump beam or bias field Bz^(\), either being in the x-direction or inthe y-direction, or any other direction which may be, for example,substantially perpendicular to the pump beam 20, for example usingHelmholtz coils 550 or 540, the ZF resonance can be observed on anoscilloscope 580 by monitoring the photodetector 90 output of themagnetometer 101, after amplification using suitable low noiseelectronics 100. The scan range of the said differing or substantiallyperpendicular field may be in the range of 0.1 times, to about 10 timesas strong as the bias field Bz^(\). Preferably, the scan range may be aslarge as the magnitude of the bias field or greater. The signal forscanning the magnetic field may be applied using a signal generator 590controlling the output of the coil driver. The scan rate may be a set ata value, preferably between 0 Hz and 1 kHz.

FIG. 7 illustrates a flow sheet for the method of identifying andoptimizing a ZF resonance for the system presented in FIG. 5. In ZF-PRMsystem, wherein a pump beam is directed through a vapor cell containinggaseous atoms, a first step, a strong magnetic field Bz^(\) 530 isapplied 701. This is accomplished by control of the 560 coils in thez-direction. After the bias field is applied, the magnetic field isscanned in a direction different from that of the pump beam which may besubstantially perpendicular to the pump beam 20, referred to herein asthe x-direction, to find the ZF resonance 702. Once the ZF resonance isidentified on the oscilloscope 570, the next step 703 is to adjust themagnetic field components in the y- and z-directions by adjusting thecurrent flowing through the Helmholtz coils 550 and 560 respectively.While observing the resonance, on an oscilloscope for example, themagnetic field values in the y- and z-direction can be adjusted bychanging the outputs from the coil driver 570 in a way that minimizesthe width of the resonance and maximizes its amplitude.

The coil driver 570 settings at which the sharpest resonance (minimumwidth and maximum amplitude) is observed corresponds with magnetic fieldbeing closest to zero in the y- and z-directions at the location of thevapor cell 70 in the magnetometer 101. The peak of the ZF resonancecorresponds with the magnetic field value closest to zero in thex-direction at the location of the vapor cell 70 in the magnetometer101.

This completes the procedure 704 for initially zeroing of the magneticfield at the location of the vapor cell 70, readying the ZF-PRM foroperation to measure external magnetic fields. To measure a field ofinterest, various prior art algorithms can be used 705 to operate themagnetometer 101, and/or to keep the magnetometer 101 locked at the ZFvalue.

An example electronic system and method for locking a ZF-PRM to a ZFresonance is schematically illustrated in FIG. 8. The magnetometer 101described in FIG. 1 is placed in a non-zero magnetic field, for exampleoutside, such that it may be exposed to the earth's magnetic field in amagnetically unshielded environment as shown in FIG. 8. The magnetometer101 may be placed in an arbitrary orientation with respect to thebackground magnetic field BG 810. As in the manual method, a strongmagnetic bias field 820 is applied using Helmholtz coil pair 850 in thedirection of the pump beam 20 (see FIG. 5), referred to herein as thez-direction. The longitudinal bias field 820 may be stronger than themaximum field BG 810 the magnetometer 101 is expected to experience, andpreferably twice as strong. The Helmholtz coils are electrically poweredby a coil driver 860.

At least one or up to three separate sinusoidal magnetic fieldmodulations, or modulation currents, may be applied to the magnetometer101 in the x-, y-, and z directions using Helmholtz coils 830, 840, and850 respectively. The electronic modulation signals may be generated bythree separate lock-in amplifiers 870, or separate and/or additionalamplifiers, and applied using a coil driver 860, which powers theHelmholtz coils 830, 840, and 850. The amplitude of the modulationcurrent is adjusted such that the peak-to-peak value of the oscillatorymagnetic field produced by each coil pair 830, 840, and 850 at thelocation of the vapor cell is about 50 nT or greater.

The sinusoidal modulation applied to each of the coils is at a differentfrequency, preferably in a range between 50 Hz and 5 kHz. In anadvantageous embodiment, the frequency of the modulation in the x- andthe y-direction is the same, but differs in phase, substantially equalto π/2 radians. The modulation in the z-direction is at a differentfrequency.

The magnetic field modulation generated by the coils, 830, 840, and 850causes the alkali spins in the magnetometer to oscillate, which in turn,modulates the intensity of the pump beam measured by the photodetector70 in the magnetometer 101. The photodetector output is amplified usinga photodiode amplifier 880. The amplified photodetector signal issubsequently fed to the lock-in amplifier 870.

Each of the three lock-in, separate, or additional amplifiers 870receive(s) the same input signal from the photodetector amplifier.However, the reference signal for demodulation for the x-axis channel ofthe lock-in amplifier is the modulation signal applied to the x-axiscoil 830, the reference signal for demodulation for the y-axis channelof the lock-in amplifier is the modulation signal applied to the y-axiscoil 840, and the reference signal for demodulation for the z-axischannel of the lock-in amplifier is the modulation signal applied to thez-axis coil 850. In this way, each of the three lock-in amplifiergenerates an independent, demodulated output signal proportional to themagnetic field in each of the three, x-, y-, and z-directions. Thedemodulation phase for lock-in amplifier in each case is adjusted in away that generates the strongest error signal. The maximum filtertime-constant is preferably adjusted to be roughly equal to the inverseof the relaxation rate of alkali spins.

After the bias field is applied, the output from at least one of thelock-in amplifier channels is fed to a control loop 890, that may in anon-limiting example be a proportional-integral-differential (PID) box,which generates feedback signals with appropriate time constants andpolarity for locking over the ZF resonance. If more than one amplifierchannel is used each may be fed to a separate control loop.

The output from the control loop 890 is fed to the coil drivers for thethree coils 830, 840, and 850. The feedback loops for the x coils 830and the y coils 840 are preferably engaged prior to engaging thefeedback for the z coils 850. Once all the control loops are engaged,the magnetic field generated by the coils 830, 840 and 850 self-convergein a way that produces a zero magnetic field environment at the locationof the vapor cell in the magnetometer.

Because the ZF resonance is significantly broadened by the bias field820, which is stronger than the ambient field, the resonance is alwayswithin the capture range of all three control loops. This is one of thesignificant benefits of our approach, which eliminates the need forcomplex algorithms for finding and locking over the ZF resonance.

The background magnetic field is measured from the input current of thecoils 830, 840 and 850 once all the feedback loops converge over thezero-field resonance. The current-to-field conversion may bemathematically calculated based on geometry of the coils, orpre-calibrated in a laboratory.

FIG. 9 illustrates a flow sheet for the method of electronically lockinga ZF resonance. This flow sheet illustrates the main steps describedbelow. In a ZF-PRM system wherein a light beam is directed at a vaporcell containing gaseous atoms, a strong magnetic bias field Bz^(†) isapplied 901. In addition to the bias field, at least on magnetic fieldmodulation is applied in one of the three orthogonal directions 902using external coils (FIGS. 8, 830, 840 and/or 850). The magneticmodulation for example may be a sinusoidal, square, or any otherperiodic oscillatory function. The frequency of the modulation field canbe in at least 10 Hz, or at least 20, or at least 50 Hz, or even atleast 100 kHz. The amplitude of the modulation preferably may be chosenin a way that generates an error signal with the steepest slope in a ZFenvironment, although other modulation amplitude settings are alsopossible, including dynamic updating of the modulation amplitude. Themodulation along each of the three axes may be the same, or differ infrequency or phase. If the modulation differs in frequency or phase thismay allow generation of independent outputs for each direction.

Using the modulated output from the photodetector 90 (FIG. 5), lock-indetection techniques described in prior art can be used to generatethree separate error signals corresponding to each of the threeorthogonal axes 903 (H. f. Dong, Fang, Zhou, Tang, & Qin, 2012). All thelock-in amplifiers may be simultaneously active, or may be sequentiallyactivated one after another in no specific order. For a bias fieldBz^(\) greater than around twice the magnitude of the background field,the resonance is always within the linear region of the error signal.

Next, using a control feedback loop, the signal lock can be engaged todrive the magnetic field values to the ZF value in all three directions904. The feedback loops for the x-coils 830 and the y coils 840 arepreferably engaged prior to engaging the feedback for the z-coils 850.Once all the control loops are engaged, the magnetic field generated bythe coils 830, 840 and 850 self-converge in a way that produces a zeromagnetic field environment at the location of the vapor cell in themagnetometer. Once all feedback loops converge, the feedback loops maybe disengaged, or they may remain engaged based on desired operationmode of the ZF-PRM, for example, to measure a sample field.

Once the background field is zeroed, a precision field environment canbe created by disengaging the feedback loop and adding a field generatedby a calibrated coil. A calibrated coil is one which has been measuredand verified to produce a known field for a given drive current orvoltage. Further a precision field environment can be created by leavingthe feedback control loop engaged and simply adding a calibrated offsetto the feedback control loop. A calibrated offset is a signal with avalue that, when added to the error signal, produces a known field.

There are many different procedures and steps that a person skilled inthe art can use to find the ZF resonance using the invention disclosedhere, and the steps described here are just one example. It isunderstood that the exact operational details may differ based on theimplementation and configuration of the ZF magnetometer, but the basicpremise of employing a bias field in the direction of the pump beam toincrease the width and the amplitude of the ZF resonance peak allowingquicker detection of ZF resonance remains applicable to allconfigurations. As an example, in a perpendicular pump-probe ZF-PRM(Romalis, Kornack, Allred, Lyman, & Kominis, n.d.), the ZF resonanceshape is a dispersive Lorenztian instead of the symmetric Lorenztianresonance R in FIG. 2 observed using ZF-PRM described in FIG. 1. Thuswhile the specific implementation of feedback loops and controlelectronics may differ, the benefits of providing a longitudinal biasfield in the same direction as the pump light source remain equallyapplicable.

While various embodiments have been described in detail, it is apparentthat modifications and adaptations of those embodiments will occur tothose skilled in the art. However, is to be expressly understood thatsuch modifications and adaptations are within the spirit and scope ofthe present disclosure.

EXAMPLES Example 1

A ZF-PRM, simply referred to here as a magnetometer, was constructed inthe following manner. The magnetometer housed a glass vapor cell in ashape approximating a cube having dimensions of about 4 mm)×4 mm×4 mm.The cell was vacuum processed and filled with an alkali vapor consistingof enriched rubidium-87 at a purity level exceeding 80% compared toother rubidium isotopes. Additionally, a buffer gas was added to thecell and the cell was sealed by melting the glass fill stem. The cellwas mounted into a custom-built, non-metallic housing. A fiber coupleddiode laser at about 795 nm was used to create a pump light beam. Thediode laser was placed about 2 meters away from the magnetometer and thepump beam was delivered to the magnetometer with apolarization-maintaining, single-mode optical fiber. A lens in themagnetometer housing was used to collimate the pump beam from theoptical fiber. Additional optical components such as polarizers,waveplates, and mirrors were used within the magnetometer housing tocondition the pump beam and direct it through the vapor cell. Aphotodiode placed after the vapor cell was used to collect thetransmitted pump beam. To increase the density of alkali atoms, the cellwas heated to a temperature over 100° C. All components in themagnetometer housing were chosen to be either non-magnetic or to havevery low residual magnetization.

We first operated the magnetometer indoors, and within a three-layermagnetic shield, to verify that the magnetometer was functioningproperly. A set of three-axis Helmholtz coils were positioned inside themagnetic shield in such a way as to surround the magnetometer housing.By modulating the current in the coil assembly, a narrow ZF resonancewith width of about 30 nT was observed. Because of the magnetic shieldsand the ZF conditions inside the shields, we were able to immediatelyfind the resonance without having to search for it. Three independentcontrol loops were set up using lock-in amplifiers which appliedfeedback to the three-axis coils. Using this setup, we were able to lockthe field to the peak of the ZF resonance in all three directions. Themodulation frequencies for the transverse directions (perpendicular tothe pump beam) were at the same frequency, but differed in phase by pi/2(90 degrees). The modulation depth was about 50 nT at the location ofthe vapor cell. With this setup we were able to reach sensitivities ofabout 5 fT/sqrt (Hz).

Example 2

The magnetometer described above was placed in a non-shielded, open-airenvironment, randomly oriented with respect to the Earth's magneticfield. Using the iterative prior art method of stepping through the biasfield, we characterized the time required to find the zero-fieldresonance of a ZF-PRM magnetometer in this un-shielded environment. Themagnetometer was oriented so that Earth's field was not substantiallyalong the direction of the pump beam. We turned on the pump beam and didnot observe a ZF resonance due to the unknown field and direction. Westarted our search for the ZF resonance by setting the applied z-biasfield to zero and stepping through values of x-bias fields and y-biasfields. After searching through values of x-bias fields and y-biasfields, we reset the x and y values and made a small change in thez-bias field. We then began our search anew by adjusting x-bias fieldsand y-bias fields for that particular z value. We iterated this processfor over an hour and were not able to find the resonance.

Example 3

To test the efficiency and ease of use of the example magnetometersystem and method of the present invention, the magnetometer, describedabove, was again located in an un-shielded, open air environment exposedto the Earth's magnetic field as in Example 2. As in Example 2, weoriented the magnetometer so that Earth's field was not substantiallyalong the direction of the pump beam. We turned on the pump beam and didnot observe a ZF resonance due to the unknown field and direction. Usingthe z-bias coils we applied a magnetic bias field, Bz^(†), in thedirection of the pump beam which is along the z-axis. The applied fieldhad a magnitude of about 100 μT which is about twice Earth's magneticfield, or in other words about twice the maximum expected field.Immediately after applying the bias field Bz^(†), a strong transmissionsignal was easily observed due to the induced broadening of theresonance and the increased strength of the resonance. By scanning thecurrent in the coil assembly, we observed a broad ZF resonance withwidth of about 100 μT. Because of the broadening of the ZF resonance, wewere able to immediately find the resonance without having to search forit. We initialized three independent control loops using lock-inamplifiers which applied feedback to the three-axis coils. The controlloops for the x- and y-bias fields were engaged first and the field waszeroed in these two directions. After that, the control loop for thez-direction was engaged which forced the z-field to zero. Using thisexample system, we were able to lock the field to the peak of the ZFresonance in all three directions. The modulation frequencies for thetransverse directions (x- and y-directions) which are perpendicular tothe pump beam were at the same frequency, about 500 Hz, but differed inphase by pi/2 (90 degrees). The modulation frequency of the z-bias wasat about 850 Hz. The modulation depth was about 50 nT, or greater, atthe location of the vapor cell. With this example system and method, wewere able to find the ZF resonance in less than about one second whichwas the time to manually engage the PID control loop for the z-biasfield. This time can be shortened to about several milliseconds or lessusing digital control.

The example system and method described herein comprising using amagnetic coil to apply a strong bias field, Bz^(†), along the directionof the pump light beam, allows near instantaneous identification of theZF resonance. Once the resonance is identified, nulling fields can beimmediately applied to initialize, or compensate for the ambient fieldsand zero, the magnetometer. Using this invention, the initializationprocedure can be completed in a matter of seconds, without any need forexternal aids or complex and time-consuming convergence algorithms. Incontrast, after over an hour's time using the iterative prior art methodof stepping through the bias field incrementally, a ZF resonance couldnot be found. The ability to quickly initialize the magnetometer byzeroing the magnetic field in its vicinity greatly increases itspractical utility of the example magnetometer.

It should be noted that the examples described above are provided forpurposes of illustration, and are not intended to be limiting. Otherdevices and/or device configurations may be utilized to carry out theoperations described herein. It can be envisioned that technologyadvances in the field may lead to variations of a magnetometer that maynot be known at this time. The method of providing a longitudinal fieldin the same direction as the light source to increase the width of thedetection bandwidth and therefore more easily and quickly compensate forand nullify the ambient magnetic field, however, will still beapplicable to such systems.

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We claim:
 1. A system for identifying and optimizing a zero-field (ZF)resonance, the ZF resonance peak having a width and a height, the systemcomprising: a. at least one pump light beam; b. a vapor cell containinggaseous atoms; c. a photodector for measuring light transmitted throughthe vapor cell; d. a strong magnetic field having a direction along thepump light beam to simultaneously increase the height and width of theZF resonance, wherein the system is configured to apply the strongmagnetic field prior to identification of the ZF resonance; e. a meansfor detecting the ZF resonance; f. a first coil, having an axis, theaxis being not aligned with the pump beam; g. an adjustable coil driver;h. at least one coil for producing a magnetic field, the coil beingselected from one or more of the first coil and one or more additionalcoils; and i. a means for adjusting a magnetic field so as to minimizethe width of the ZF resonance peak and maximize the height of the ZFresonance peak.
 2. The system of claim 1, wherein the system is orientedalong any arbitrary direction with respect to a background magneticfield.
 3. The system of claim 1, wherein the system is not placed withina magnetic shield.
 4. The system of claim 1, wherein the strong magneticfield applied along the direction of the pump light beam is at leastabout as strong as a background magnetic field.
 5. The system of claim1, wherein the strong magnetic field applied along the direction of thepump light beam is stronger than a background magnetic field.
 6. Thesystem of claim 1, wherein the strong magnetic field applied along thedirection of the pump light beam is weaker than a background magneticfield.
 7. The system of claim 1, wherein the gaseous atoms in the vaporcell are selected from a group consisting of rubidium, cesium,potassium, sodium, and helium.
 8. The system of claim 1, the systembeing configured such that the first coil, having an axis, the axisbeing not aligned with the pump beam, is adjustable across a scan range,the scan range being from at least about 0.1 times as strong as themagnetic field applied along the pump light beam, to about 10 times asstrong as the magnetic field applied along the pump light beam.
 9. Thesystem of claim 1, further enhanced for precision offset fieldgeneration, further comprising a means for adding a calibrated field.10. The system of claim 9, wherein the calibrated field is generated byone or more coils selected from a group consisting of coils that areinternal to the magnetometer and coils that are external to themagnetometer.
 11. A system for locking a zero-field paramagneticresonance magnetometer (ZF-PRM) to a ZF resonance, the systemcomprising: a. at least one pump light beam; b. a vapor cell containinggaseous atoms; c. a photodector for measuring light transmitted throughthe vapor cell; d. a strong magnetic field having a direction along thepump light beam to simultaneously increase the height and width of theZF resonance, wherein the system is configured to apply the strongmagnetic field prior to identification of the ZF resonance; e. a meansfor detecting the ZF resonance; f. a current modulator to generate amodulation current, the current modulator attached to a first coil, thecoil having an axis, the axis being not aligned with the pump beam; g.an adjustable coil driver; h. at least one coil, the coil being selectedfrom selected from one or more of the first coil and one or moreadditional coils; i. a means for generating at least one error signal;and j. a means for engaging at least one control loop that controls thecoil having an axis, the axis being not aligned with the pump beam, thecontrol loop configured to minimize at least one field component that istransverse to the pump light beam.
 12. The system of claim 11, whereinthe system is oriented along any arbitrary direction with respect to abackground magnetic field.
 13. The system of claim 11, wherein thesystem is not placed within a magnetic shield.
 14. The system of claim11, wherein the strong magnetic field applied along the direction of thepump light beam is at least about as strong as the background magneticfield.
 15. The system of claim 11, wherein the strong magnetic fieldapplied along the direction of the pump light beam is stronger than thebackground magnetic field.
 16. The system of claim 11, wherein thestrong magnetic field applied along the direction of the pump light beamis weaker than the background magnetic field.
 17. The system of claim11, wherein the gaseous atoms in the vapor cell are selected from agroup consisting of rubidium, cesium, potassium, sodium, and helium. 18.The system of claim 11, wherein the modulation current is applied to atleast two coils transverse to the pump light beam, such that at leasttwo error signals are generated.
 19. The system of claim 11, wherein themodulation current is applied to at least three coils transverse to thepump light beam, such that at least three error signals are generated.20. The system of claim 11, further comprising: a. a means for changingthe amplitude of at least one modulation current; and b. a means foroptimizing at least one error signal.
 21. The system of claim 11,further comprising a means for engaging at least one control loop thatminimizes the field component in the direction along the pump beam, thesystem configured to minimize the field component in the direction alongthe pump beam subsequent to engaging at least one control loopconfigured to minimize at least one field component that is transverseto the pump light beam.
 22. The system of claim 11, further enhanced forprecision offset field generation, further comprising: a. a means fordeactivating one or more control loops; and b. a calibrated fieldgenerator.
 23. The system of claim 22, wherein the calibrated fieldgenerator comprises at least one coil.
 24. The system of claim 11,further enhanced for precision offset field generation, furthercomprising a means for adding one or more calibrated offsets to one ormore control loops.